1. Field of the Invention
The present invention relates to a tensioning device for maintaining desired tightness within a flexible inextensible drive element in a system such as a camshaft drive for an internal combustion engine.
2. Disclosure Information
Devices incorporating flexible inextensible drive elements such as fiber-reinforced belts or metal linked chains have traditionally employed tensioning devices. In order to be effective, such devices must damp excess motion of the chain or belt without imposing unreasonably high hub loads upon either the driving member, such as a crankshaft of an engine, or other driven members such as idlers or camshafts. Unfortunately, if tension is not adequate, the chain or belt may skip a tooth, causing unacceptable engine operation or, in extreme cases, engine damage. If, on the other hand tension within the belt or chain is too great, excessive wear will occur within engine bearings such as crankshaft main bearings or camshaft bearings.
It is known to construct a chain tensioner, for driving a camshaft of an internal combustion engine, with the tensioning force being provided by a piston which is pressurized by engine lubricating oil. Such devices usually employ a controlled leakage factor to exert a force of appropriate magnitude upon a tensioner shoe in the direction tending to tension the chain properly, while damping out excess oscillations of the chain. Such tensioners also typically include a ratcheting device which prevents the tensioner shoe from releasing from the chain beyond a predetermined amount when oil pressure drops off, as when the engine is shut down. Unfortunately, such known devices are prone to overextension of the ratcheting tensioner device during startup operating at lower ambient temperatures, which may cause the ills described above as accompanying excess tension in the chain.
It would be desirable to provide a tensioner which not only prevents excessive falloff in chain tension when the engine is shut down, but which also prevents overtightening or overtensioning of a chain or belt during operation of the engine.